It is well known that incandescent light bulbs are a very energy inefficient light source—about 90% of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are by a factor of about 10 more efficient, but are still less efficient than a solid state semiconductor emitter, such as light emitting diodes, by a factor of about 2.
In addition, incandescent light bulbs have a relatively short lifetime, i.e., typically about 750-1000 hours. Fluorescent bulbs have a longer lifetime (e.g., 10,000 to 20,000 hours) than incandescent lights, but they contain mercury, not an environment friendly light source, and they provide a less favorable color reproduction. In comparison, light emitting diodes have a much longer lifetime (e.g., 50,000 to 75,000 hours). Furthermore, solid state light emitters are a very clean “green” light source and can achieve a very good color reproduction.
Accordingly, for these and other reasons, efforts have been ongoing to develop solid state light devices to replace incandescent light bulbs, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide improvement with respect to energy efficiency, color rendering index (CRI Ra), luminous efficacy (lm/W), color temperature, and/or duration of service, especially for indoor applications.
A semiconductor light emitting device utilizes a blue light emitting diode having a main emission peak in the blue wavelength range from about 400 nm to 490 nm and a luminescent layer containing an inorganic phosphor that absorbs the blue light emitted by the blue LED and produces an excited light having an emission peak in a visible wavelength range from green to yellow (in the range of about 530 nm to 580 nm) having a spectrum bandwidth (full width of half maximum, simply refer to FWHM) of about 80 nm to 100 nm.
Almost all the known light emitting semiconductor devices utilizing blue LEDs and phosphors in combination to obtain color-mixed light of the emission light from the blue LEDs and excitation light from the phosphors use a YAG-based or silicate-based luminescent layer as phosphors. These solid state light devices have typically a white color temperature of about 5000 K to 8500 K with a low color rending index Ra of about 60˜70. This type of white solid state light device is not desirable for some applications, like indoor applications, which require a warm white color temperature of about 2700 K to 3500 K with a high color rending index Ra above 80.
A conventional solid state warm white light device is realized by adding orange or red phosphors into yellow or green phosphors to adjust the color temperature to less than about 3500 K and improve the color rendering index. However, there are low luminous efficacy issues caused by: a) multi-phosphors self-absorption loss of the photons excited from the green and orange phosphor particles; and b) Stoked-shift loss from blue-to-red wavelength conversion.
Thus, there remains a need for an improved warm white solid state light device that overcomes mixed-multi-phosphors self absorption loss and Stoked-shift loss from blue-to-red wavelength conversion.
There is also a need to further improve luminous efficacy in order to produce higher electrical-to-optical energy conversion efficiency with a good thermal dissipation design for a compact incandescent bulb replacement device and compete with fluorescent bulbs for high volume and cost effective commercial and residential applications.
There is also a need to improve color mixing uniformity from multi-colors semiconductor light emitting device in order to produce a color uniform light from a solid state lighting device for lighting applications.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified need could be fulfilled.